Method for detecting the load on an internal combustion engine

ABSTRACT

A method for determining engine load of a spark ignition internal combustion engine. The method includes the steps of charging an ignition transformer to a maximum ignition charge and initiating an ignition discharge between electrodes of the spark plug. The ignition transformer is then charged to a predetermined diagnostic charge which is less than said maximum ignition charge. The diagnostic charge is discharged at a time of the combustion cycle where pressure in the combustion cylinder is dependent on engine load. The duration diagnostic discharge is then monitored and subsequently correlated to engine load. In this manner, the spark plug is used as a feedback element in determining engine load.

FIELD OF THE INVENTION

The present invention generally relates to a automotive ignition systemfor an internal combustion engine. More particularly, this inventionrelates to a coil-on-plug ignition transformer which is capable of beingfired according to an algorithm to perform various engine diagnosticprocedures. The spark plug mounted ignition system of the presentinvention therefore operates as a feedback element of the engine controlsystem.

BACKGROUND AND SUMMARY OF THE INVENTION

In order to initiate combustion of an air/fuel mixture within aninternal combustion engine, a spark ignition system generates a highvoltage arc across the spark plug electrodes at the appropriate time inthe engine operating cycle. The onset of the arc across the spark pluggap is timed to occur at a predetermined number of degrees of crankshaftrotation, usually before the piston has reached top dead center (TDC).

If the spark timing is properly set, the combustion process initiated bythe spark plug action will cause a pressure increase to develop withinthe combustion chamber that will peak just shortly after TDC during thepiston's power stroke. If the spark is initiated too late in theoperating cycle (retarded timing), the pressure developed within thecombustion chamber will not be efficiently converted by the engine intowork. On the other hand, if the spark is initiated too early in theoperating cycle (advanced timing), extremely high and potentiallydamaging pressures and temperatures may result. The pressure andtemperature increases associated with advance timing are also notefficiently converted by the engine into a useful work output.

Excessive advanced timing can also lead to the occurrence of severalother types of combustion chamber phenomena. One such phenomena isauto-ignition of the end gases and another is pre-ignition.

Auto-ignition is a condition where the end gases (the unburnt portion ofthe fuel-air mixture initially ignited by the movement of the flamefront) explode spontaneously as a result of the cylinder temperature andpressure becoming too high for the type of fuel being burned in theengine. In response to the sudden release of energy, the cylindertemperature dramatically increases and the cylinder pressure fluctuates,alternately rising and falling, as a pressure wave travels back andforth across the combustion chamber. When caused by auto-ignition of theend gases, the rapid pressure and temperature fluctuations are seen tooccur after TDC. If the rate at which energy is released throughauto-ignition is high enough, the exploding gases will cause thecylinder walls to vibrate resulting in audible engine noises, includingthe distinctive sound known as "pinging".

Many engine developers believe that a mild degree of auto-ignition isdesirable because it generates turbulence within the combustion chamber,which hastens the combustion process, at a critical time when the normalflame kernal is in the process of being quenched. Slight auto-ignitionhas also been found to reduce the amount of unburnt hydrocarbonsremaining after the completion of the spark-triggered ignition process.By utilizing the energy released when the hydrocarbons are burned duringmild auto-ignition, it follows that lower hydrocarbon emissions andimproved fuel economy can be realized.

Because of the benefits stated above, among others, engine designersoften seek to calibrate ignition systems so that the spark advance isclose to the threshold of auto-ignition. However, excessiveauto-ignition must be avoided since it leads to higher combustionchamber temperatures and is counter productive. In fact, these elevatedtemperatures can heat the spark plug electrodes to the point where theywill initiate the combustion process independently of the occurrence ofa spark. This phenomena is pre-ignition.

Pre-ignition, which can cause significant engine damage includingperforation of the piston, is characterized by the occurrence ofextremely high cylinder temperatures and pressures near TDC. The audiblesound associated with pre-ignition is produced by the action ofauto-ignition and, when extreme, referred to as "knock". Generally, itcan be stated that auto-ignition leads to pre-ignition and,subsequently, that pre-ignition leads to further auto-ignition.

A number of factors influence the spark timing threshold which generatesauto-ignition. Some of these factors include, inlet air temperature,engine speed, engine load, air/fuel ratio and fuel characteristics.Because accurate control of the spark timing is a significantcontributor to engine performance, numerous types of engine controlsystems have been developed. These control systems typically employ amicroprocessor based closed-loop spark timing control system whichsimultaneously measures a number of parameters, such as exhaustcomposition, coolant temperature, and the occurrence of spark knock viatransducers. The resulting data is then processed to set the enginetiming near a predicted auto-ignition threshold.

The knock detectors typically used in engine control systems arepiezoelectric transducers which sense the intense vibration caused byspark knock. When used in the environment of an internal combustionengine, however, these transducers may not be selective enough todistinguish the slight vibration produced by incipient auto-ignitionover the normal amount of engine vibration. For this reason, thesedetectors are typically not capable of sensing, particularly at highengine speeds, the threshold of auto-ignition. An engine control systemis therefore needed which is capable of detecting incipientauto-ignition and which enables more precision in setting the sparktiming in a closed-loop system.

Other characteristics found in ignition systems and considered to beundesirable include, but are not limited to: excessive spark plugelectrode wear; the inability to fire fouled spark plugs; poor coldweather starting; poor exhaust emissions during cold engine starting andrunning; the remote generation of high voltages in the enginecompartment by the ignition system; the routing and distribution of highvoltages over considerable lengths of ignition wire; and the generationof significant mounts of electro-magnetic radiation within and aroundthe ignition system, as well as the vehicle, during operation of theengine.

It is therefore an object of the present invention to provide an enginecontrol and ignition system which overcomes the limitations anddisadvantages of known systems.

It is also an object of this invention to provide an ignition systemwhich is capable of performing various engine diagnostic procedures soas to operate as a feedback element of the engine control system. Inparticular, the invention operates as a non-invasive combustion chambermonitor through the utilization of the ignition transformer and thespark plug as the feedback elements.

The present invention has as further objects the providing of a methodfor determining engine load, a method for detecting engine misfire and amethod for detecting auto-ignition of the end gases.

Another object of the invention is to provide a coil-on-plug ignitiontransformer which is capable of charging, tiring and retiring the sparkplug at short, repeatable intervals as programmed into the enginecontrol system.

One feature of this invention is that it eliminates the various problemsassociated with the distribution of high voltages throughout theignition system. Another feature of the present invention is that itreduces the amount of electro-magnetic radiation generated by theignition system around the engine and the vehicle itself.

Reduced spark plug electrode wear is another feature as well as theability to fire badly fouled spark plugs.

A still further feature of the invention is enhanced cold weatherstarting capabilities of an internal combustion engine and theminimization of exhaust emissions which occur during cold starting andrunning. A related feature is the extension of the air/fuel ratio towardthe lean limit which helps to further reduce emissions and improve fueleconomy during normal engine operation.

SUMMARY OF THE INVENTION

Recent research, some of which has been performed by the assignee of thepresent invention, has indicated that combustion within an internalcombustion engine can be improved by initiating the burning process witha spark of the type known as a breakdown discharge. The breakdown sparkhas characteristics quite different from those generated by conventionalautomotive ignition systems and responds differently to differentconditions within the combustion chamber. This realization has led tothe development of the present invention, an ignition control systemhaving components which are capable of exploiting the characteristics ofthe breakdown spark so as to enable the performance of various enginediagnostic procedures using the spark plug itself as a feedback elementof the engine control system.

The ignition process has been characterized as consisting of threedistinct phases; the breakdown phase; the arc phase and the glow phase.The initial phase, the breakdown phase, is characterized by high current(typically 50-200 amperes (A)) which results from the energy stored inthe spark plug capacitance (typically 10-15 picofarad (pF)) dischargingthrough the arc. The breakdown phase typically lasts about a nanosecond(ns). The second phase, the arc phase, occurs when the arc current isbetween 0.1 and 1.0 A and the arc voltage is about 180 volts (v). Thedischarge current remains in the arc phase for approximately 100μs. Theglow phase occurs when the arc current drops below 0.1 milliamperes (mA)and the voltage across the spark plug electrodes goes to 50O v.

These three phases, the breakdown, arc and glow phases, have been foundto reliably initiate combustion of the air/fuel mixture when theair/fuel ratio is respectively twenty-one to one, eighteen to one andsixteen to one. If the breakdown phase is exploited, it follows that thelean limit can be extended and numerous benefits realized.

As mentioned above, the present invention details an ignition and enginecontrol system which is not only capable of firing the spark plug, butwhich is also capable of performing diagnostic functions. Specifically,one aspect of the present invention details the ignition and enginecontrol system itself. Another aspect details the methods for performingvarious diagnostic procedures. A further aspect of this invention is alow impedance ignition transformer, mounted directly on the spark plug,which enables both of the above. The transformer's low impedanceaugments the capabilities of the engine control system's microprocessorunit (MPU) making it possible for the MPU to use the spark plug tomonitor a number of engine conditions including misfire, auto-ignitionand engine load.

The ignition and engine control system of the present invention includessix principal components not counting the engine itself. These are anengine controller (which has inputs that monitors various engineparameters), a MPU (which is programmed to carry out various routinesbased on the inputs to the engine controller), ignition or coil drivercircuit, an ignition transformer, a spark plug and current dischargedetection circuitry, all of which are described in greater detail below.

The design of the ignition transformer provides for a short chargingtime and an intense secondary current of short duration (approximately0.5-1 A, decaying to zero in approximately 100 μs) that reliablyinitiates stable combustion. This is achieved while deriving energydirectly from the vehicle's 12ν power supply and eliminating the needfor an expensive 12νDC to 250νDC converter.

Because of the intensity and duration spark, as well as the shortcharging time of the transformer, the present transformer configurationenables the elimination of the ignition system's high voltagedistribution system and also makes possible the rapid, multi-firing ofindividual spark plugs by the engine control system. Previously,multi-firing ignition systems have had to rely on a fixed countdowncounter or a natural resonance within the ignition circuitry toretrigger the firing. In a standard ignition system, the charging timefor the primary, and therefore the time necessary for re-firing of thespark plug, is about 3000 μs. Relatively slow in terms of the durationof the engine operating cycle. The present invention, however, isdesigned to multi-fire based on algorithms programmed into the enginecontrol system itself and has the capability of retiring the spark plugsat 200 μs intervals.

Under hard to ignite conditions, it has been found that the multi-firingof the spark plug during the combustion event is beneficial to thecombustion process. According to the present invention, multi-firing isprogrammed to occur only under hard-to-ignite mixture conditions such asthrottle tip-ins, cold starts, idle and at combinations of light loadsand low rpms. By not multi-firing under other conditions, an extensionin the life of the ignition components is realized, particularly in thespark plug electrodes.

Since spark plug electrode wear is directly proportional to the timeover which the arc current flows, electrode wear can be reduced byapplying a higher intensity current over a shorter duration. Asmentioned above, when current flowing between the spark plug electrodesis above 100 mA, the voltage is about 180ν. Below 100 mA, however, thevoltage rises to about 500ν. When accelerated by a 500ν differential,the electrons and charged particles being exchanged between the sparkplug electrodes penetrate the electrode surfaces more vigorously thanwhen accelerated by a 180ν differential.

In a standard flyback ignition coil system, the electrons and chargedparticles are driven for well over 1,500 μs at the 500ν differential.This results in significant electrode wear. In the low impedance systemof the present invention, the energy level that results in the peakvoltage across the spark plug electrodes is intense, about 22 kilovolts(kν), but it is reached approximately 4 μs after the transformer primaryhas been switched off and the overall time spent above the 500νdifferential is typically less than 20 μs. While the increased intensityof the spark better ensures stable combustion, its significantly shorterduration minimizes spark plug electrode wear. This is beneficial sinceit makes it possible to reduce the diameter of the spark plug electrodesthemselves. It is well known that spark plug electrodes having a smallersize and mass will minimize quenching of the initial kernel of burninggases and produce more stable combustion.

The intensity and short duration of the spark plug arc current isadvantageous and beneficial in several other regards. These benefitsinclude, but are not limited to: more stable combustion; reduced energyconsumption by the ignition process; lower overall exhaust emissions;extending operation of the engine further toward the lean limit;extended catalytic converter life; a reduction in arc current time andspark plug electrode wear; the increased ability to fire fouled sparkplugs; enhanced cold weather starting and running capabilities; areduction of cold start exhaust emissions; an elimination of highvoltage routing about the engine; and a reduction in electromagneticradiation generation in and around the vehicle.

As mentioned above, the system of the present invention can be used todetect the misfiring of a cylinder in the engine. After the fullycharged ignition transformer has been switched off generating a maximumsecondary voltage across the spark plug electrodes and starting thecombustion process, while the crankshaft and the combustion cycle arestill near TDC, the MPU causes the ignition transformer to develop apredetermined applied voltage at the spark plug gap. If combustion hasalready been initiated, the combination of temperature and pressure inthe area of the spark plug will enable the applied voltage to conductacross the electrodes. If the cylinder has misfired, the predeterminedlevel of applied voltage at the spark plug gap will not be high enoughto cause the spark plug electrodes to conduct. As a result of theapplied voltage not being spent in a secondary current discharge, anegative voltage excursion is reflected back into the primary. Theelectronic switch of the primary winding is monitored by the detectioncircuitry and the engine control system and, if this negative voltageexcursion is detected, the system records that misfire has occurred. Ifthe misfire repeats for a successive combustion cycles, the MPU andengine controller can be programmed to shut the cylinder down preventingunburnt hydrocarbons from being released in the exhaust emissions andreducing fuel consumption. In an attempt to curb exhaust emissions,various states are enacting laws that require that a misfiring cylinderbe shut down. One such law goes into effect in California in 1996.

The present invention can also be used to detect auto-ignition of theend gases and set the engine timing at the threshold of auto-ignition.In using the spark plug to detect whether auto-ignition of the end gasesis occurring, the MPU causes the ignition transformer to rapidly dutycycle at a predetermined voltage. This is done at a point in thecombustion cycle where knock is expected to occur (typically aftertop-dead-center (ATDC)). The duty-cycle period is calculated from analgorithm stored in the MPU of the engine controller and is a functionof various engine parameters including engine load, engine speed, andcharge temperature.

If normal combustion conditions are occurring in the cylinder at thetime of duty cycling, the current resulting from the duty cycling willnot be transferred across the plug gap, but will instead be reflectedback through the primary as a negative voltage excursion. The negativeexcursion can again be detected at the high side of the electronicswitch by the detection circuitry and the engine controller. Ifauto-ignition is occurring, the resulting temperature and pressure wavespresent within the cylinder will correspond with one or more of theapplied duty cycle voltage potentials enabling it to conduct across theelectrode gap. As a result, not all of applied voltages will have acorresponding reflected negative voltage excursion. By monitoring theprimary for a missing negative voltage excursion, auto-ignition of theend gases can be recognized and detected by the engine controller. Usingthis information regarding the occurrence or non-occurrence ofauto-ignition, the engine controller can progressively step the ignitiontiming so that threshold of auto-ignition is maintained.

Additionally, the uniqueness of the present ignition transformerfacilitates the measurement of the spark plug breakdown voltage duringthe combustion cycle. It is the magnitude of this parameter (whichreflects the relationship between the combustion pressure, temperatureand fuel concentration) that provides a non-intrusive indication of theengine's performance or load. By enabling monitoring of the engine load,the ignition and engine control system of the present inventioneliminates the need for expensive manifold absolute pressure (MAP)sensors. Knowing that the cylinder pressure is proportional to theengine load, the spark plug breakdown voltage can be directly correlatedto the engine load in view of Paschen's Law. At an "interrogate" time orcrank angle position ATDC, where other variables such as spark advanceand the air/fuel ratio are no longer an influence on the cylinderpressure, the breakdown voltage is determined by firing the spark plugand measuring the time over which the transformer inductive currentdischarge. In view of the transformer's known characteristics, thedischarge time is then correlated by the engine controller intobreakdown voltage to determine the cylinder pressure and, ultimately,the engine load.

All of the above is made possible by the short charging and dischargingtime of the ignition transformer, the ignition and detection circuitryand the control software programmed into the MPU and engine controller.In the time it takes a conventional ignition transformer to perform asingle charge and discharge, the ignition transformer of the presentinvention is capable of initiating combustion, recharging and retiring amultiple number of times to perform the diagnostic procedures.

Intended to operate within the spark plug well of the engine, theflyback transformer of the present invention incorporates a torodialdesign that eliminates the flow of magnetic flux inside the cylinderdefining the spark plug well. This makes the present ignitiontransformer largely insensitive to eddy current loading and is a majorreason for the decreased production of electro-magnetic radiation.

Having a restricted diameter, the ignition transformer itself includes acylindrical core whose length can be varied to provide the necessarycross sectional area in the transformer core. The core is positionedwithin a dielectric bobbin and the primary and secondary of thetransformer are wound around both the bobbin and the core. The woundcore and bobbin is then positioned within a housing whose lower end isconfigured to receive the high side terminal of a spark plug. The sparkplug itself can be of a standard design or can be modified to reflectthe ability to use smaller electrodes with the present invention.

The electronics of the ignition and engine control system are controlledby engine controller which monitors input signals from the cam and crankspeed sensors, as well as the vehicle ignition signal. These inputsallow the engine controller and the MPU to calculate engine speed andposition. As a result of these calculations, the MPU calculates andsends output signals at the proper time, based on its programmedalgorithm, to coil driver circuits which charge and trigger the ignitiontransformer. The MPU utilizes the detection circuitry to monitor thecombustion cylinder and determine the engine load and/or whether a knockor misfire condition exists. Depending on the existing conditions, theMPU signals and alerts other circuits or modules of the engine to takethe appropriate measures.

Additional benefits and advantages of the present invention will becomeapparent to those skilled in the art to which this invention relatesfrom the subsequent description of the preferred embodiments and theappended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing the general components of anignition and engine control system embodying the principles of thepresent invention;

FIG. 2 is a perspective view with portions broken away showing theignition transformer of the present invention positioned on the sparkplug of an internal combustion engine;

FIG. 3 is a longitudinal sectional view of a portion of an ignitiontransformer embodying the principles of the present invention;

FIG. 4 is a perspective view of the core, bobbin, primary and secondarywindings as provided by the present invention;

FIG. 5 is a top plan view of the core, bobbin, primary and secondarywindings as seen in FIG. 4;

FIG. 6 is a perspective view of the transformer core;

FIG. 7 is a longitudinal sectional view of a second ignition transformerincorporating the principles of the present invention;

FIGS. 8(a) and (b) are a graphical representations of the primarycharging current and the secondary discharge voltage with respect totime;

FIGS. 9(a)-(d) are graphical illustrations of the pressure andtemperature at the spark plug during both a normal combustion event anda misfire event, as well as the applied voltages and reflected voltagesoccurring in the transformer during both events;

FIGS. 10(a)-(c) graphically illustrate the pressure and temperature inthe cylinder during a normal combustion event as well as the applied andreflected voltages in the ignition transformer during knock detection;

FIGS. 11 (a)-(c) are graphical illustrations of the pressure andtemperature in the cylinder during auto-ignition of the end gases aswell as the applied and reflected voltages in the ignition transformer;

FIG. 12 is a graphical illustration of the cylinder pressure relative tocrank angle position for various engine loads;

FIG. 13 is a graphical illustration of the breakdown voltage relative tothe inductive current discharge time; and

FIG. 14 is a schematic illustration of the coil driver circuits,ignition transformer and the detection circuits utilized in the presentinvention.

DETAILED DESCRIPTION Of THE PREFERRED EMBODIMENT

Referring now to the drawings, an ignition and engine control systemembodying the principles of the present invention is generallyillustrated in FIG. 1 and designated at 20. The system includes anengine controller 22 and an MPU 24 which spends most of its timeexecuting a main program loop that performs various engine functionswhich are relatively non-critical from an engine timing standpoint. Therate at which these functions must be repeated is also relatively slowin comparison to the engine cycle itself. This generally means thatthese "non-critical" functions can be performed asynchronously from theengine combustion events.

Fuel injection and ignition events, however, must be preciselysynchronized to the engine cycle. To accomplish this, the enginecontroller 22 and MPU 24 are programmed to service interrupts that aretriggered by timing pickups or speed sensors 26 mounted on the engine 28relative to a flywheel 30 on the crankshaft and/or a pulley 32 on thecamshaft. The interrupts produced by the timing pickups 26 load a timingelement of the MPU 24 which creates real time control signals for thefuel injectors and ignition coil drivers at the correct instant and forthe correct duration during the combustion cycle. The engine controller22 is also be coupled to various other engine parameters including thevehicle ignition signal.

Using the results of the above calculations, the MPU 24 outputs signalsat the proper time through an ignition or coil driver circuit 34 causingan ignition coil or transformer 36 to begin charging directly from thevehicle's 12ν power supply. The ignition transformer 36, which ismounted directly onto a spark plug 38 and is known as a coil-on-plugtransformer 36, is charged until its core becomes saturated. At theappropriate number of engine degrees before top dead center (BTDC), theMPU 24 then causes a high speed switching transistor of the coil drivercircuit 34 to open, shutting off the current in the transformer primary.If conditions are right within the engine cylinder, the secondarycapacitance of the transformer 36 will discharge in a high voltagecurrent across the spark plug 38 gap and initiate combustion. After theignition transformer has been scheduled to fire, the MPU 24 runs througha series of programmed algorithms designed to cause multi-firing of thespark plug or perform various engine diagnostic procedures. Ifdiagnostic procedures are being performed, the MPU 24 utilizes thedetection circuitry 40 as further outlined below.

The ignition transformer 36 of the present invention is a very lowimpedance device which, by design, is capable of generating asignificant secondary voltage (about 25kν) which peaks in approximately2-4 μs and decays to zero in approximately 100 μs. Since the transformer36 will fully charge and saturate its core in about 100 μs from thevehicles 12μ power supply, this means the transformer 36 is capable ofbeing retired at 200 μs intervals.

Previously, to create signals for repetitively operating the coil drivercircuit 34 or for multi-firing an ignition transformer and spark plug at200 μs intervals, numerous timing interrupts would have had to beenserviced by the engine controller 22 and MPU 24 for each refiring of thespark plug. This, however, would result in excessive interrupt loadingof the MPU 24 and would create a significant number of timing conflicts.With excessive interrupts being present, the main program the MPU 24would be disrupted at a high frequency during a large percentage of itsexecution time resulting in interrupts being nested within one another.The multiple timing conflicts would require the MPU 24 to service morethan one interrupt at a time in order to generate the required controlsignals. The MPU 24, however, can only execute one interrupt at a time.

In the present invention, the MPU 24 is directed by the enginecontroller 22 to send signals to the coil driver circuit 34 according toa specific algorithm programmed into the MPU 24. Thus, the need forservicing a multitude of interrupts is eliminated because of the shorttime necessary to re-fire the transformer 36.

The ignition and engine control system 20 of the present inventionutilizes a specially designed spark plug mounted ignition coil ortransformer 36 as a feedback element in the engine control system 20. Inaddition to its feedback functions, the ignition transformer 36 providesan intense, short duration (less than 100 μs) secondary current thatreliably initiates combustion, even when the spark plug is badly fouled,and promotes spark plug longevity.

The uniqueness of the ignition transformer 36 provides for anon-intrusive indication of engine performance by facilitating themeasurement of the spark plug breakdown voltage, a parameter whosemagnitude reflects the relationship between the combustion pressure,temperature, and fuel concentration. In general, the relationshipbetween the pressure, temperature and electrode gap is defined byPaschen's Law which states: ##EQU1## where P is the pressure; d is theelectrode spacing; T is the temperature; and K₁ and K₂ are constants.

The voltage level that is generated by the ignition transformer 36 isdirectly related to the magnitude of the primary winding current, whichis a function of charging time, at the time the ignition transformer isswitched. In the present invention, the primary current that generatesthe maximum secondary voltage is typically reached in a charge time of100 μs when the voltage applied to the primary winding is 12ν. A chargetime of less than 100 μs will therefore result in a secondary voltagethat is less than the maximum. In other words, the shorter the chargetime, the lower the secondary voltage of the ignition transformer 36.

Referring now to FIGS. 2 and 3, the spark plug mounted or coil-on-plugignition transformer 36 of the present invention is generallyillustrated therein. The physical dimensions of the ignition transformer36 are dictated by the design of the engine 28 itself. To enablemounting directly on the spark plug 38 itself, the ignition transformer36 must be able to fit within the diameter of a spark plug well 41 ofthe engine 28. While this specific design criteria differs from oneengine version to the next, the principles of the present invention willbe applicable to the entire range of spark plug well diameters. Thelength limit of the ignition transformer 36 is determined by theclearance between the engine 28 and the hood of the vehicle (not shown).The length of the ignition transformer 36 can therefore be adjusted toaccommodate the required cross sectional area of its core, as determinedby the various other transformer parameters.

The ignition transformer 36 of the present invention includes a magneticcore 42 which is received in a dielectric bobbin 44. Perhaps best seenin FIGS. 4-6, the core 42 is substantially cylindrical and includesportions which define an air gap 46 that extends the length of the core42. In order to provide a very efficient transformer 36, the retentivityof the core is required to be a very small percentage of its maximumflux density. When the magnetizing force (expressed in ampere turns) isremoved from the core 42 of the transformer 36 by switching off theprimary current, the residual magnetic flux in the core 42 rapidlydecreases. The voltage generated in the secondary winding of thetransformer 36 by the collapse of the primary current is directlyproportional to the number of turns in the secondary and the magnitudeof the change in the core flux and is inversely proportional to the timerate of change in the core flux. Mathematically stated: ##EQU2## wheree_(sec) is the secondary voltage, L is the inductance of the secondarywinding, ##EQU3## is the time rate of change in the core flux, and thenegative sign (-) indicates that the core flux is decreasing.

To comply with the mathematical performance requirements, themanufacturing tolerances of the core 42 must be such that the crosssectional area of the core 42 is substantially constant. While the limiton the overall length of the transformer 36 and the length of thetransformer core 42 is determined by the clearance between the engine 28and the hood of the vehicle, the dimensional limit on the core's insidediameter is determined by the access requirements of the machine whichwinds the wire of the primary and secondary windings onto the core 42.The physical limits on the outside diameter of the core 42 aredetermined by, not only the diameter of the spark plug well 41, but alsothe dielectric strength of the material from which the bobbin 44 ismade.

The bobbin 44 which receives the core 42 includes an inner cylindricalsleeve 48 and an outer cylindrical sleeve 50. Each of the sleeves 48 and50 further include a radial flange at one end which extends over theends of the core 42 to encase it within the bobbin 44. In theillustrated embodiment, the inner sleeve 48 is provided with anoutwardly directed radial flange 49 at its distal end, while the outersleeve 50 is provided with an inwardly directed radial flange 51 at itsproximal end. The outer diameter of the inner sleeve 48 and the innerdiameter of the outer sleeve 50 are dimensioned so that the core 42 isin surface-to-surface contact with the inner and outer sleeves 48 and50. Preferably, the bobbin 44 is made of a material having a highdielectric strength, such as one of the well known plastics.

Referring now to FIGS. 3-5, the primary and secondary windings 52 and 54of the present ignition transformer 36 can be seen. The windings 52 and54 are wound longitudinally about the core 42 and bobbin 44 so as toextend along the interior surface defined by the inner sleeve of thebobbin 44, over one of the longitudinal ends of the bobbin 44, along theexterior surface defined by the outer sleeve 50 and across the opposinglongitudinal end. Facilitating the efficiency of the transformer 36, theprimary winding 52 consists of a lesser number of turns of largerdiameter wire than the secondary winding 54 and is located on the bobbin44 immediately over the air gap 46 defined in the core 42. The secondarywinding 54 of smaller diameter wire substantially covers the remainderof the core 42 and bobbin 44. The combination of the windings 52 and 54provides the core 42 and bobbin 44 with a generally torodial shape thatis best seen in FIG. 4.

After the windings 52 and 54 are positioned over the bobbin 44 and thecore 42, the wound assembly is positioned within a cavity 55 definedwithin a cylindrical, insulative housing 55. The inboard or proximal endof the housing 56, generally designated at 58, is provided with threads60 which engage a similarly threaded adapter 62. The adapter 62 isconstructed from a conductive metal and is configured to allow theignition transformer 36 to engage the mounting nut 63 of the spark plug38.

The proximal end 58 of the housing 58 has mounted therein, in a threadedengagement, an ignition terminal 64 which is adapted to electricallyengage the high side terminal 66 of the spark plug 38. To ensureengagement between the ignition terminal 64 of the transformer 36 andthe high side terminal 66 of the spark plug 38, the ignition terminal 64may be provided with a biased contact element or spring 68 thatpositively engages the high side terminal 66 and is secured by solderingor other bonding techniques within a seat 70 of the ignition terminal64. The biasing of the contact element 68 not only ensures thatelectrical contact will be made with the spark plug terminal 66, butalso provides the transformer 36 with a range over which it is capableof engaging the spark plug 38.

The transformer 36 is also provided with an annular seal 72 of rubber orother suitable material in the housing 58 and is positioned around thehigh side terminal 66 and contact element 68. The seal 72 preventsmoisture and dirt from entering between the spark plug 38 and theignition transformer 38 and fouling the electrical contact therebetween.

The ignition terminal 64 of the transformer 38 is connected by a lead 74to the high side 75 of the secondary winding 54. The low side 77 of thesecondary winding 54 is connected by a second lead 76 to the adapter 62which electrically engages the mounting nut 63 and grounds the sparkplug 38. The primary winding 52 has its ends 81 connected to terminals82, on the distal end of the transformer 36, which couple thetransformer 36 to the ignition circuitry 34 and the remainder of theengine control and diagnostic system 20.

Substantially filling the remainder of the cavity 55 defined by thehousing 56 is a suitable dielectrical material. While numerous otherconsiderations may dictate the specific nature of the dielectric fillingmaterial, it is believed that various types of materials could beutilized with satisfactory results. For example, the dielectric fillingmaterial may be a preformed solid material fitted within the housing.Another would include a setable dielectric material poured into thehousing and allowed to subsequently harden. Still another variety wouldinclude a liquid dielectric material poured into and sealed within thehousing. Additionally, it is believed that combinations of the abovecould be used.

As seen in FIG. 7, another embodiment of the transformer 36 of thepresent invention is illustrated therein with elements common to theprevious embodiment being given like designations. In this secondembodiment, the cavity of the transformer 36 is filled with a dielectricliquid and a central insulative post 83 is positioned to extend intosubstantially through the bore of the bobbin 44 from an end cap 84 whichseals the dielectric liquid within the housing 56. To further ensure theintegrity of the seal between the end cap 84 and the housing 56, anO-ring 85 can also be provided at the engagement of the housing 56 andthe adaptor 62 for the same purposes. In substantially all otherrespects, the transformer 36 of the second embodiment is the same asthat of the first.

As an illustrative example of the present invention, the following ispresented for a preferred embodiment of the ignition transformer 36 whenthe diameter restriction on the transformer 36 is 24 mm. The transformer36 includes a core 42 made of a material having the characteristicsdescribed above and which typically experiences a change in flux fromabout 14,000 to 500 Gauss. One such material, know as METGLAS, isproduced by the Allied Signal Corporation and sold as Alloy 2605 TCA.The core 42 has an overall length of about 3.15 inches, an outerdiameter of about 0.67 inches, an Inner diameter of about 0.48 andincludes a longitudinal air gap which is about 0.005 inches wide. Thebobbin 44 is made from a material having a dielectric strength of about680 volts/mil. One such material is a polyphenylene sulfide manufacturedby the Hoechst Celanese Corporation and sold under the tradenameFORTRON. The inner and outer sleeves 48 and 50 have a radial thicknessesof about 0.13 inches and 0.11 inches, respectively. Three turns of #24wire are provided for the primary winding 52 and 210 turns of #40 wireare provided for the secondary winding 54. The dielectric liquid istransformer oil. The resulting transformer 36 exhibits an inductance ofabout 12.6 μH (microHenrys) and, when connected to the vehicle's 12νpower source, develops a maximum primary current of 50A in about 100 μsand generates a secondary peak volt of about 25 kν which decays to zeroin about 100 μs.

In use, the primary 52 of the ignition transformer 36 is coupled to theignition circuit 34. More particularly, the high side of the primarywinding 52 is connected to a high speed, high current switchingtransistor 101 whose function is to switch the charging current on andoff in response to a signal generated by the MPU 24 (an Intel 87C51FA8-bit microcontroller in the preferred embodiment discussed above) asdetermined by its programmed algorithm. To fully charge the transformer36, the primary winding 52 is connected through the coil driver andignition circuit 34 across the vehicle's 12ν power supply forapproximately 100 μs. At the end of this time period, the current withinthe primary will have peaked at 50A, a value at which the transformercore 42 will have become saturated. As seen in FIG. 8, upon the 50Acurrent 86 being abruptly shut-off by the high speed switchingtransistor 101, a voltage 87 will be induced in the transformer'ssecondary 54 which will peak in 2-4 μs at approximately 25 kν and decayto zero in about 100 μs. The low impedance of the ignition transformer36 results in the voltage being efficiently transferred to theelectrodes 82 of the spark plug 38. Also because of the transformer'slow impedance, the time necessary to reach a breakdown voltage levelthat will cause an arc to form across the electrodes 82 is a fraction ofa microsecond. Under normal engine operating conditions, the spark plug38 will conduct in the range of 7-12 kν. If the primary current in thetransformer 36 is limited by reducing the charging time, the maximumsecondary voltage that is generated when the primary 52 is turned offwill also be limited.

Referring now to FIG. 9, when conditions exist in the combustion chamberof the engine 28 that cause the spark plug 38 not to conduct the energystored in the capacitance of the secondary 54, the system 20 of thepresent invention can be used to detect this misfiring of the cylinder.During normal combustion, the MPU 24 causes the coil driver circuit 34to initiate an increasing coil charging current in the primary 52 of thetransformer 36. Once the transformer 36 has been fully charged, thecurrent is switched off, designated at 88, by the switching transistors101 generating a maximum secondary voltage and beginning ignition withinthe combustion chamber. If normal combustion has been initiated, thepressure and temperature at the spark plug electrodes 80 will generallyincrease as designated by curves 90 and 92.

To determine whether combustion or misfire has occurred, the MPU 24 isprogrammed to cause the ignition transformer 36 to initiate a chargingcurrent 94 and develop a lower, predetermined applied voltage at thespark plug gap. This is timed so as to occur just before top dead center(BTDC). During normal combustion, the combination of pressure andtemperature at the spark plug electrodes 80 will be sufficient to permitthe lower applied voltage to conduct across the electrodes 80. As aresult, the energy stored in the secondary capacitance will bedischarged across the electrodes and will not be reflected back into theprimary 54 of the transformer 36. (see FIG. 9(c)). During misfire,however, the pressure and temperature at the electrode 80 will not havesufficiently increased, as designated by curves 96 and 98, to enable thelower applied voltage 94 to conduct. As a result of this, the energy ofthe secondary capacitance will be reflected back into the primary 52 ofthe transformer 36 and appear as a negative voltage excursion 100 whichcan be detected on the high side of the switching transistor 101.

As seen in FIG. 14, the detection circuit 40 of the present inventionincludes a subcircuit 102 for detecting negative voltage excursions 100.For each cylinder of the engine 28, the sub-circuit 102 includes a diode104 whose cathode is attached to the one high side of the switchingtransistor 101. In this manner, a single detection sub-circuit 102 canbe used to monitor all of the engine's cylinders. For the sake ofclarity, only two of the transistors 101 and diodes 104 are illustratedin FIG. 14. The diodes 104 feed any negative excursion through thesub-circuit 102 where the signal is conditioned and passed to a negativethreshold reference comparator 106. The comparator 106 outputs acorresponding signal to the MPU 24 which processes the signal based onits programmed algorithm and, if necessary, shuts down a misfiringcylinder.

Detecting auto-ignition of the end gases (knock) uses the same basicapproach as detecting misfire. Referring now to FIGS. 10 and 11, anormal combustion cycle and a knock combustion cycle are respectivelyillustrated therein. During normal combustion, the pressure within thecylinder 108, as indicated by curve 108, does not begin to significantlyincrease until ADTC. This is also true for the temperature within thecylinder, as indicated by curve 110. However, during a knock combustioncycle, pockets of exploding end gas cause pressure waves, which travelback and forth across the combustion chamber within the cylinder, alongwith a dramatic increase in the cylinder temperature. This typicallybegins to occur around 10° ATDC. The pressure and temperature curves ofthe knock combustion cycle are respectively indicated as curves 112 and114 in FIG. 11, with the pressure fluctuations being designated at 116and the temperature rise being designated at 118.

During the time period when knock is most likely to occur (typicallyaround 10°-20° ATDC), the MPU 24 duty cycles the coil driver circuit 34and current 120 going to the transformer 36 so as to produce a series ofapplied voltages. Because of the combination of pressure 108 andtemperature 110 during normal combustion, the level of the appliedvoltage 120 is chosen so that the spark plug 38 will not subsequentlyconduct during normal combustion. As a result, a negative voltageexcursion 122 is reflected back into the primary 52. As seen in FIG.10(c), a negative voltage excursion 122 will be present for each appliedvoltage 120 during a normal combustion cycle. The sub-circuit 102 feedsthis information as an input to the MPU 24 where it is processed andpassed on to the engine controller 22, which utilizes this informationto advance the spark timing toward the threshold of auto-ignition.

In a "knock" combustion cycle (FIG. 11), the applied voltages 120 areagain generated when the combination of pressure fluctuations 116 anddramatic temperature increase 118 are expected to occur. By applying aseries of voltages 120 over this time frame, the chances that at leastone of the applied voltages 120 will correspond with a decreasedpressure fluctuation and allow the applied voltage 120 to discharge inan arc across the spark plug gap is increased. As a result, acorresponding negative voltage excursion 122 will be absent. If one ormore of the reflected voltages 122 are missing, designated at 124, inresponse to a correspondingly applied voltage 120, the MPU 24 will sensethis through the detection sub-circuit 102 and send the appropriatesignals to the engine controller 22 so that the spark timing can becorrespondingly stepped toward eliminating knock. By alternatelyadvancing and retarding the timing of the engine as described above, theengine controller 22 is capable of maintaining the spark timing at thethreshold of auto-ignition.

The ignition transformer 36 of the present invention can also be used toexploit the value of the breakdown voltage to determine engine load. Toreliably determine the breakdown voltage level, the relationship betweenthe charging energy (the energy required to charge the distributedcapacitance of the secondary 54 up to the breakdown voltage level) andthe distributed energy (the energy dissipated by the spark plug arccurrent) is used. This relationship is represented by the equation:

    ε.sub.Ts =1/2*CV.sub.BD.sup.2 +1/2*EI.sub.p *t

where VBD is the breakdown voltage at the spark plug; C is thedistributive capacitance of the secondary circuit; E is the arc currentvoltage at the electrodes; I_(p) is the peak arc current at the sparkplug electrodes 80; t is the arc current discharge time which variesinversely with the breakdown voltage; and ε_(Ts) is the total energyavailable to the secondary circuit. By solving the above equation forthe breakdown voltage, the breakdown voltage can be expressed as afunction of time with the remaining parameters all being known valuesdependent on the specific design of the transformer 36.

Referring now to FIG. 12, during the monitoring of engine load, thespark plug breakdown voltage is determined at an "interrogate" time orcrank angle position 126 where the effect of other variables, such asthe temperature, the air/fuel ratio and the spark advance, are no longeran influence on the cylinder pressure. This is most likely to occurwithin the range of about 20°-50° ATDC, depending on the particularengine. The value of the breakdown voltage at the interrogate crankangle position 126 is therefore directly proportional to the cylinderpressure, which in turn is indicative of the engine load. Three pressurecurves, which relate to a heavy load 128, a light load 130 and an idleload 132, are shown in FIG. 12.

At the "interrogate" crank angle position 126, the coil driver circuit34 initiates a current 134 (in FIG. 9) that charges the primary 52.Specific to the engine load or pressure then present within thecylinder, the energy stored in the secondary 54 will begin dischargingat a specific breakdown voltage across the spark plug gap and willcontinue discharging for a corresponding time period.

While direct measurement of the breakdown voltage is problematic,measuring the duration of the inductive current discharge, and relatingthis time to the breakdown voltage (see FIG. 13), is more easilyperformed. This is accomplished by a load or second detectionsub-circuit 136 of the detection circuit 40. Again, a single sub-circuit136 is used to monitor all of the engine cylinders.

Once the spark plug 38 has started to conduct the secondary current, asecond set of diodes 138, whose anodes are attached to the high side ofthe switching transistor 101, detect the positive voltage associatedwith the secondary current flow and feed the associated voltage into theload detecting sub-circuit 136 of the detection circuit 40. As long asthe arc current is flowing, the voltage at the high side of theswitching transistor 101 will be significantly above the 12 νdc powersupply of the vehicle. The load detection sub-circuit 136 outputs apulse to the MPU 24 having a length which corresponds to the duration ofthis elevated voltage and the length of time over which the inductivecurrent discharges. The MPU 24 correlates the inductive pulse width intothe breakdown voltage which, using Paschen's Law, can be correlated tothe pressure in the cylinder and engine load. The MPU 24 then outputsthis information to the engine controller 22 so that the spark timing,air/fuel ratio and other ignition and engine control parameters can beappropriately modified.

More specifically, the inductive pulse width measurement is started atthe end of the ignition dwell and is done by monitoring the reflectionwhich occurs in the primary 52 during the secondary discharge. Thereflected signal in the primary 52 and an auto-tracking referencesignal, which compensates for variations in the power supply voltage,are biased and filtered at appropriate levels to provide an accuratemeasurement of the inductive phase. These signals are then fed to acomparator 140 which detects the inductive current reducing to zero ornear zero. Once the inductive current has decayed, a signal from thecomparator 140 is fed to a flip-flop 142 which has also received aninput indicating the end of the ignition dwell. This enables theflip-flop 142 to output a signal representing the inductive pulse widthto the MPU 24. The MPU 24 then correlates the inductive pulse width tobreakdown voltage allowing the cylinder pressure and engine load to bedetermined.

While the above description constitutes the preferred embodiments of thepresent invention, it will be appreciated that the invention issusceptible to modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

We claim:
 1. A method for determining load in a combustion cylinder of aspark ignition internal combustion engine, said method comprising thesteps of:charging an ignition transformer to an energy level resultingin a predetermined ignition voltage; initiating an ignition dischargebetween electrodes of a spark plug positioned within said combustioncylinder, said ignition discharge occurring at a predetermined number ofdegrees of engine rotation before piston top-dead-center; charging saidignition transformer during the same ignition cycle to produce adiagnostic charge resulting in a diagnostic voltage; applying saiddiagnostic voltage to said electrodes of said spark plug at apredetermined number of degrees of engine rotation; discharging saiddiagnostic voltage across said electrodes of said spark plug in aninductive current discharge; measuring the duration of said inductivecurrent discharge across said electrodes of said spark plug; correlatingthe duration of said inductive current discharge to pressure within thecombustion cylinder at the time of said inductive current discharge: andcorrelating the pressure within the combustion cylinder at the time ofsaid inductive current discharge to engine load whereby said spark plugis utilized as a feedback element in determining engine load.
 2. Amethod for determining engine load as set forth in claim 1 wherein saidpredetermined number of degrees of engine rotation is during a period ofthe combustion cycle when pressure within the engine cylinder is notsubstantially dependent on the ignition timing.
 3. A method fordetermining engine load as set forth in claim 1 wherein saidpredetermined number of degrees of engine rotation is during a period ofthe combustion cycle when pressure within the engine cylinder is notsubstantially dependent on the air/fuel ratio.
 4. A method fordetermining engine load as set forth in claim 1 wherein saidpredetermined number of degrees of engine rotation is during a period ofthe combustion cycle when pressure within the engine cylinder is notsubstantially dependent on the temperature within the cylinder.
 5. Amethod for determining engine load as set forth in claim 1 wherein saidpredetermined number of degrees of engine rotation is after pistontop-dead-center.
 6. A method for determining engine load as set forth inclaim 1 wherein said predetermined number of degrees of engine rotationis within the range of 20-50° degrees after piston top-dead-center.
 7. Amethod for determining engine load as set forth in claim 1 wherein saidpredetermined number of degrees of engine rotation is about 45° degreesafter piston top- dead-center.
 8. A method for determining engine loadas set forth in claim 1 wherein said diagnostic voltage increases inmagnitude with respect to time as said ignition transformer is charging.9. A method for determining load in a combustion cylinder of a sparkignition internal combustion engine having an engine controller coupledto a spark plug through a diagnostic circuit whereby said controllerutilizes said spark plug as a feedback element in determining engineload, said method comprising the steps of:charging an ignitiontransformer to an energy level generating a predetermined ignitionvoltage; initiating an ignition discharge between electrodes of saidspark plug positioned within said combustion cylinder, said ignitiondischarge occurring at a predetermined number of degrees of enginerotation before piston top-dead-center; initiating a signal based onengine timing inputs to begin charging of said ignition transformer forperforming an engine load diagnostic procedure; charging said ignitiontransformer during the same combustion cycle to produce a diagnosticcharge resulting in a diagnostic voltage; applying said diagnosticvoltage to said electrodes of said spark plug at a predetermined numberof degrees of engine rotation after piston top- dead-center whenpressure within the combustion cylinder is substantially only dependenton engine load; increasing said diagnostic voltage in magnitude overtime as said ignition transformer charges; discharging said diagnosticvoltage in an inductive current discharge across said electrodes of saidspark plug; measuring the duration of said inductive current dischargewith said diagnostic circuit; outputting from said diagnostic circuit tosaid controller a signal having a pulse width corresponding to theduration of said inductive current discharge; correlating said pulsewidth to pressure within the combustion cylinder at the time of saidinductive current discharge; and correlating said pressure within thecombustion cylinder at the time of said inductive current discharge toengine load whereby said spark plug is utilized as a feedback element indetermining engine load.
 10. In a method for determining engine load asset forth in claim 9 wherein said controller initiates said signal basedon timing inputs.
 11. In a method for determining engine load as setforth in claim 9 wherein said magnitude of said diagnostic voltageincreases continuously over time.
 12. In a method for determining engineload as set forth in claim 9 wherein said controller is programmable andhas an algorithm programmed therein to correlate said pulse width topressure.
 13. In a method for determining engine load as set forth inclaim 9 wherein said controller is programmable and includes analgorithm programmed therein to correlate pressure within the combustioncylinder to engine load.
 14. In a method for determining engine load asset forth in claim 9 wherein said predetermined number of degrees ofengine rotation after piston top-dead-center is within the range of20-50° degrees after piston top-dead-center,
 15. In a method fordetermining engine load as set forth in claim 9 wherein saidpredetermined number of degrees rotation after piston top-dead-center isabout 45° degrees after piston top-dead-center,
 16. A method fordetermining load in a combustion cylinder of a spark ignition internalcombustion engine having an engine controller coupled to a spark plugthrough a diagnostic circuit whereby said controller utilizes said sparkplug as a feedback element in determining engine load, said methodcomprising the steps of:charging an ignition transformer to an energylevel resulting in a predetermined ignition voltage; initiating anignition discharge between electrodes of said spark plug positionedwithin said combustion cylinder, said ignition discharge occurring at apredetermined number of degrees of engine rotation before piston top-dead-center; initiating a signal based on engine timing inputs to begincharging of said ignition transformer for performing an engine loaddiagnostic procedure; charging said ignition transformer during the samecombustion cycle to produce a diagnostic charge resulting in adiagnostic voltage; applying said diagnostic voltage to said electrodesof said spark plug at a predetermined number of degrees of enginerotation after piston top-dead-center when pressure within thecombustion cylinder is substantially only dependent on engine load;increasing said diagnostic voltage in magnitude over time as saidignition transformer charges; discharging said diagnostic voltage in aninductive current discharge across said electrodes of said spark plug;measuring the duration of said inductive current discharge, saiddiagnostic circuit measuring the duration of said inductive currentdischarge by monitoring the time over which the high side of an ignitiontransformer associated with said spark plug is above 12 vdc; outputtingto said controller a signal having a pulse width corresponding to theduration of said inductive current discharge; correlating said pulsewidth to pressure within the combustion cylinder at the time of saidinductive current discharge; and correlating said pressure within thecombustion cylinder at the time of said inductive current discharge toengine load whereby said spark plug is utilized as a feedback element indetermining engine load.